1. Field of the Invention
The invention concerns a photochemical system for coating luminescent screens of color television picture tubes containing, as photosensitive substances, polyvinyl alcohol and a salt of chromic acid.
2. Description of the Prior Art
The luminescent coating on the luminescent screen of color television picture tubes consists of a mosaic of various luminescent substances. Generally, these are red-, green- and blue-emitting luminescent substances.
In a known manner, the luminescent substances are applied to the glass screen, in a suspension forming a photochemical system, in a series of consecutive steps, whereupon each coating applied to the screen is exposed to light in precisely defined areas. The exposed areas of the coating remain adherent to the glass screen, while the rest is washed away in a further step.
The application of such a photochemical system entails problems that affect the quality of the luminescent coating. Thus the darkness reaction, i.e., the hardening of the suspension in the absence of light, and the exposure time dependent on the luminescent substances, lead to fluctuations in the size of the luminescent coating areas.
To overcome both problems, it was proposed to add to the polyvinyl alcohol layer certain quantities of ethan diol and propane triol, of which one will delay the darkness reaction and the other will accelerate the light reaction.
The drawbacks of this known solution of the problem lie in the facts that a special additive substance is required for each aforementioned problem and that this solution is not entirely and without further problems suitable for all luminescent substances.
In the mass production of goods that must meet exacting requirements with respect to a constant level of quality, the emphasis is on making the auxiliary means and methods within the framework of production as simple and trouble-free as possible.